[Big'un]

Ok when i first crank my truck and start movin it makes a rattling sound almost directly under my feet ...... also when im goin down the road and get in it and it downshifts it does it until i let out of it .... its hard to describe the sound i wish i cud a video .... it gets on my nerves so bad! I thought it might have been a loose heat sheild but i checked them all and there tight .... could it be the cat? but its not under my feet its more like under the backseat floorboard .

HELP!